1. Field of the Invention
The present invention relates to a display device using charged particles that move between electrodes upon the application of an electric field in a dispersion medium. More particularly, the present invention relates to a display device in which disperse systems of charged particles in a dispersion medium are encapsulated in individual microcapsules, and images are formed by controlling the direction of movement of the charged particles by the application of a controlling electric field.
2. Related Art of the Invention
Japanese Patent Publication 52-28554 and numerous other reports describe image display devices for moving particles between electrodes by the application of an electric field and forming images on the screen. These display devices are constituted such that disperse systems of particles in a liquid dispersion medium are placed between a pair of electrodes faced to each other at least one of which is transparent, an electric field is applied between these electrodes, and the particles in the dispersion medium are controlled depending on their polarity so as to be attracted to or repelled by the transparent electrode plate to display the desired images.
The following materials may commonly be employed as the liquid dispersion medium for the disperse systems: aliphatic hydrocarbons, aromatic hydrocarbons, alicyclic hydrocarbons, halogenated hydrocarbons, various esters, alcohol-based solvents, and various other oils. These may be used independently or as appropriate mixtures to which emulsifiers, surfactants, or the like have been added in an appropriate manner. In addition, the following may be used as the particles: conventional colloid particles, various organic or inorganic pigments or dyes, metal powders, fine glass or resin powders, and the like.
With a structure in which disperse systems are merely placed between electrodes, however, display defects tend to occur due to particle aggregation and the adhesion of particles to the electrodes. Structures have therefore been proposed in which perforated spacers shaped as porous materials or meshes provided with a large number of through holes are disposed between a pair of electrodes, and the disperse systems are broken up into a discontinuous arrangement, making it possible to stabilize display manners.
In electrophoretic display devices provided with perforated spacers, the disperse systems are placed in each of the through holes after the perforated spacers have been interposed between the electrodes. However, there is a problem that it is very difficult to uniformly fill each of the numerous through holes with the disperse systems.
As a method of overcoming the aforementioned disadvantage, a method in which disperse systems of particles in a liquid dispersion medium are individually encapsulated in microcapsules is described in Japanese Patent Application Laid-Open No. 64-86116. Because this method can prevent particles from aggregating or adhering to the electrodes, it suppresses the formation of so-called display defects and makes it easier to fill the spaces between the electrodes with the dispersion medium.
However, the above-described method, when the particles in the microcapsules have nonuniform diameters, is still unable to prevent display defects from occurring during image formation, because there is a possibility that those particles will be very large or very small, or the amount of these particles encapsulated in the microcapsules will be very large or very small. Since the particles with nonuniform diameters vary apparent charges in the liquid dispersion medium, the migration rate of the particles varies during the application of low voltage, making it difficult to drive the device with low power.
It is difficult to independently control the color and charge polarity of particles contained in the liquid dispersion medium when these particles are conventional colloid particles, various organic or inorganic pigments, or the like. Therefore, when two types of particles having different colors and charge polarities are encapsulated within microcapsules, it is also difficult to control the direction of migration of each type of particle to form images having the desired colors.
In a case of a system in which a single type of particle is caused to move in microcapsules to form an image, when the controlling electric field is switched off, the particles in the capsules float to the top or sink to the bottom, depending on the relation of their specific gravity to that of the liquid dispersion medium. The controlling electric field must therefore be constantly applied in order to preserve the images on the screen.